TESTING THE 12AU7 ON VARIOUS TUBE TESTERS INTRODUCTION Take a batch of tube testers, all in good working order and carefully calibrated following the manufacturer’s instructions. Separate them by type— mutual conductance testers in one group, emissions testers in another. Wouldn’t you think that if you ran the same tube through all of them, you’d get comparable readings from all of the testers within their respective groups? That would be logical— and wrong. As it happens, each individual tester applies unique conditions to a tube under test, so readings from tester to tester, even on the same model, will not necessarily be comparable. Not only that: readings obtained on mutual conductance testers other than laboratory models usually won’t match those published in standard tube manuals. Why? Because manuals assume a set of operating conditions that most testers simply cannot duplicate.